1) Field of the Invention
The invention relates to a transparent heat-sealable, biaxially oriented polyester film having at least one heat-sealable layer of copolyester which contains ethylene 2,6-naphthalate units, to the use thereof and to a process for the production thereof.
2) Prior Art
Because of their advantageous physical properties, biaxially oriented polyester films have a wide variety of applications, e.g. as support for magnetic tapes or as dielectric for capacitors. Because they are effective barriers to oxygen and have good optical properties, polyester films are suitable as packaging material. For many of the packaging applications, it is desirable for the films to be heat-sealable and have a high seal seam strength, even at low sealing temperatures. A further requirement of such films is that they can be produced cost-effectively and without significant problems and that they can easily be further processed.
In the prior art, the materials used for the heat-sealable layers of heat-sealable polyester films are either
crystalline or partially crystalline, with a low melting point (e.g. polyethylene, propylene homopolymer or copolymers of ethylene and propylene) or PA1 amorphous, with a glass transition temperature T.sub.g, measured by DSC, which is lower than that of the associated base layer.
Polyester films which are produced by the first-named method have the advantage of a low sealing temperature. The minimum sealing temperature (also termed sealing initiation temperature) of such films is about 100.degree. C., making the films extremely suitable for processing on high-speed packaging machines. A disadvantage of these films, however, is that the heat-sealable layers/films have to be applied to the polyester film by lamination using adhesion promoters. This additional process step raises costs and in some circumstances may be uneconomic. In addition, the optics of such bonded materials are unsatisfactory. A further disadvantage is that the polyethylene of the heat-sealable layer generally contains lubricants, such as erucamide. When the film is reeled, the erucamide may be transferred onto the opposite surface of the polyester film and thus make this side unsuitable for subsequent metalizing.
In heat-sealable polyester films which are produced by the second method, the amorphous heat-sealable layers can be applied onto the base layer of polyester by means of coextrusion. This method is more cost effective than lamination. The amorphous heat-sealable coextruded layers generally consist of copolyesters containing isophthalic acid and terephthalic acid, as described, for example, in GB-A 1 465 973 and EP-A 0 035 835.
GB-A 1 465 973 describes a coextruded polyester film having two layers, one layer of which consists of copolyesters containing isophthalic acid and terephthalic acid and the other layer of which consists of polyethylene terephthalate. The patent gives no useful indications concerning the sealing performance of the film. The lack of pigmentation means that the film cannot be produced by a reliable process (the film cannot be wound up) and that the possibilities for further processing of the film are limited.
EP-A 0 035 835 describes a coextruded heat-sealable polyester film where, in the heat-sealable layer, particles whose mean size exceeds the heat-sealable layer thickness are added in order to improve winding and processing performance. The particulate additives form surface protrusions which prevent undesired blocking and sticking to rolls and guides. The choice of particles having diameters greater than the heat-sealable layer thickness impairs the sealing performance. The patent does not give an indication of the sealing temperature range of the film. The seal seam strength is measured at 140.degree. C. and is in a range from 85 to 120 N/m (from 1.275 N/15 mm to 1.8 N/15 mm film width).
EP 0 432 886 describes a coextruded multilayer polyester film which has a first layer on which is arranged a heat-sealable layer and a second layer on which is arranged an acrylate layer. The heat-sealable outer layers here may also consist of copolyesters containing isophthalic acid and terephthalic acid. The coating on the reverse side gives the film improved processing performance. The patent gives no indication of the sealing temperature range of the film. The seal seam strength is measured at 140.degree. C. For a heat-sealable layer thickness of 11 .mu.m, a seal seam strength of 761.5 N/m (11.4 N/1 5 mm) is given.
EP 0 515 096 describes a coextruded multilayer heat-sealable polyester film which contains a further additive in the sealable layer. The additive may, for example, contain inorganic particles and is preferably distributed onto the film during its production, in an aqueous layer. By this method, the film is claimed to retain its good sealing properties and to be easy to process. The patent gives no indication of the sealing temperature range of the film. The seal seam strength is measured at 140.degree. C. and is more than 200 N/m (3 N/15 mm). For a heat-sealable layer of 3 .mu.m thickness, a seal seam strength of 275 N/m (4.125 N/15 mm) is given.
A data sheet on Melinex.RTM. 850 (IC), a copolyester-coated polyester film, gives a sealing temperature range of from 120 to 200.degree. C. for this film, the seal seam strength being 4.8 N/15 mm (measured at a sealing temperature of 140.degree. C., a sealing pressure of 2.8 bar and a sealing time of 2 sec). A disadvantage of this film is its relatively high sealing temperature and its low seal seam strength, in particular when short sealing times and low sealing pressures are used.
In comparison with non-heat-sealable polyester films, heat-sealable polyester films have, inter alia, inferior optical properties and slip properties, especially at elevated temperatures.
In comparison with non-heat-sealable polyester films, the production and processing of heat-sealable polyester films is clearly inferior when their sealing-layer copolymers have a glass transition temperature lower than that of the base layer. Because of the lower glass transition temperature, the heat-sealable surface of the film tends to block and stick during production and during processing. Problems in the production process and in the conversion process occur significantly more frequently than with a non-heat-sealable standard polyester film. Thus, for example, during longitudinal stretching in film production, heated rolls heat the amorphous film to a temperature which is somewhat higher than the glass transition temperature of the base layer. The heat-sealable layer, which has a lower glass transition temperature than that of the base layer, is already tacky at this temperature. For this reason, there are more film breaks and/or complete production stoppages during longitudinal stretching of heat-sealable polyester films. The method using the abovementioned raw materials for the film is therefore restricted in application. Printing or metalizing of the film are similarly affected. In both processes, the film is heated and therefore the tendency of the film to stick, for example to guiding rolls or during winding, is greatly increased.
It is known that copolymers of polyethylene terephthalate and polyethylene 2,6-naphthalate form an amorphous phase in the range from about 15 to 85% by weight of polyethylene 2,6-naphthalate as a proportion of the entire mixture (Plastics Technology, May 1995). There is no indication in this publication that these amorphous copolymers can be used for producing heat-sealable polyester films, in particular for producing heat-sealable polyester films having a low sealing initiation temperature. A person skilled in the art would not expect that copolymers of this type would give heat-sealable polyester films, since the glass transition temperature T.sub.g of these copolymers is higher than that of those of the PET base layer. Rather, these facts would deter the person skilled in the art from using such polymers to produce heat-sealable films.